When magnetic recording disks are rotated in a disk drive, the magnetic head on a slider is caused to ride, or glide, on a cushion of air, which is commonly referred to as an air bearing, at a predetermined height slightly above the surface of the disk. As the density of hard disks continues to increase, the height of the air bearing, or the so-called "glide height", is forced to become lower and lower. This creates problems for air flow. As a result, most of the hard disks are "textured" on the surfaces thereof, so as to improve the flow of air current.
Texturing the disk surface, which creates bumps on the disk surface, however, generates another type of problem. If the heights of some of the bumps, or peaks, are greater than the glide height of the hard disk slider, the magnetic head may be inevitably damaged. Thus, after a production run, the magnetic disks must be tested of their glide height, which is defined as the minimum permissible glide height of the hard disk slider or the maximum height of the bumps, so as to ensure that they will not cause damages to the hard disk slider. Asperities that are high enough to impact the magnetic head slider during disk operation can cause failure of the drive. Thus, production run disks with asperities above a predetermined height must be identified and discarded.
Typically, as part of the hard disk fabrication process, production run magnetic disks are tested using a calibrated slider for their asperities. The calibrated slider rides on an air bearing at substantially the same distance from the disk surface as would the magnetic head slider in the disk drive. Typically, the calibrated slider can bend and move like a rigid body as a result of impacts with the asperities. When an asperity on the production run disk exceeds a predetermined tolerance, the calibrated slider will detect and indicate the asperity on the test result, signaling the production personnel to discard that particular disk. An improperly calibrated test slider may fail to detect many unsatisfactory production run disks, or, equally unacceptable, it may reject too many production run disks that actually provide acceptable quality.
Several methods have been developed in the past to make calibration disks, or bump disks, for the calibration of a test slider. Typically, these methods involve generating bumps on a calibration disk which emulate the undesirable asperities of a production run disk. In U.S. Pat. No. 5,062,021, the content thereof is incorporated by reference, it is disclosed a method of making crater shaped bumps on a magnetic disk using a laser pulse. The diameter of these crater shaped bumps is 0.8 mils and the height of the peripheral ridge of the crater above the nominal surface of the disk is in the range of 0.5 to 0.8 microinches.
In U.S. Pat. No. 5,236,763, the content thereof is incorporated by reference, it is disclosed a method of making elliptical crater shaped bumps by impinging multiple pulses of a laser beam on a printing roller. The major and minor diameters of the elliptical crater shaped bumps are 82 .mu.m and 60 .mu.m, respectively.
In U.S. Pat. No. 5,528,922, the content thereof is incorporated by reference, it is disclosed a method of employing laser pulses to make bumps on a calibration disk which can be used for calibrating a piezoelectric transducer slider. Crater shaped bumps having a diameter in the range of 10 to 25 .mu.m and a peripheral ridge with a height in the range of 75 to 120 nm are made by impinging two or more pulses of laser energy on the same location of a calibration disk.
The above inventions provide certain advantages; however, the method that has been prevalently used in the industry for making calibration bump disks involves the use of a metal mask technique. FIG. 1 shows an illustrative flowchart diagram of the key steps of the metal mask technique for making calibration bump disks. First, a substrate 9 is obtained. A metal mask 90 having an aperture 92 is provided covering the substrate 9, except the area covered by the aperture 92. The areal dimension of the aperture 92 determines the areal size of the final bump to be made. A metal layer 93 is then formed on the surface of the metal mask 90, typically by a sputter means. A portion of the metal layer 93 is also formed inside the aperture 92. After the metal mask 90 is removed, a bump 91 is revealed in the place corresponding to where the aperture 92 was. After rinsing, overcoating, lubricating, testing, etc, a bump disk is obtained.
In the past, the metal mask technique has provided very good results for making precision bump disks. However, as market demand on the density of hard disks takes constant quantum jumps, the metal mask technique has become somewhat inadequate. As the density of hard disks increases, the glide height of the magnetic head slider becomes lower and lower. In the conventional metal mask technique, the depth of the aperture is typically 50 .mu.m (i.e., the thickness of the metal mask), with a diameter of about 120 nm. Present day hard disk test runs require a calibration disk with bump height of 0.0457 .mu.m. Due to the large difference (more than 1,000-fold) between the depth of the aperture (50 .mu.m) and the intended height of the standard bump (0.0457 .mu.m), the region near the periphery of the aperture does not receive the same amount of sputtering ion during the sputtering step. As a result, the bump so formed does not have a flat top as intended. Rather, the calibration bump would show a convex, non-flat, shape at least near its periphery. In other words, the calibration bump does not have a uniform height. As the intended height of the calibration bump becomes smaller, the inadequacy of the conventional metal mask technique becomes more profound. Similar problems are encountered when the intended diameter of the calibration bumps has to be reduced.